Inorganic fine particles, inorganic raw material powder, and method for production thereof

ABSTRACT

The present invention provides new process for preparing inorganic fine particles, which suppresses agglomeration and adhesion of particles due to heat when preparing powder from a raw material liquid such as a slurry of zirconia hydrate fine particles in order to obtain inorganic raw material powder having sharp particle size distribution. Furthermore, the present invention provides a new process for preparing inorganic fine particles, which can make the chemical structure homogenous among the produced particles and inside the particles even in a multi-component system. The present invention provides a process for preparing inorganic fine particles, which comprises heating and applying impulse waves to a raw material liquid.

TECHNICAL FIELD

The present invention relates to inorganic fine particles, inorganic rawmaterial powder and preparation processes thereof. As the process forpreparing inorganic fine particles, the present invention relates to theprocess of preparing fine particles of zirconia hydrate and as theprocess for preparing inorganic raw material powder, the presentinvention relates to the process of preparing zirconia raw materialpowder from fine particles of zirconia hydrate. Also, the presentinvention relates to inorganic fine particles and inorganic raw materialpowder prepared by the processes.

BACKGROUND ART

Ceramic powder has conventionally been prepared by precipitatinghydrates comprising neutralized hydroxides, neutralized coprecipitatedhydroxides and hydrolyzates of metal salt and composites thereof, makingthe precipitate into powder by a drying method such as still drying andspray drying, and thereafter calcinating and pulverizing.

As a process for preparing zirconia fine powder, JP-2000-327416discloses the method of obtaining raw material powder by dissolving anaqueous solution of salt such as yttrium or cerium as a secondarycomponent in an aqueous solution of zirconium salt, calcinating theaqueous solution in the presence of oxygen after rapid drying, wetpulverizing the calcinated article and then drying. Also, JP-A-5-246720discloses the method of obtaining raw material powder by spray dryingand calcinating a slurry obtained by hydrolysis or neutralizationprecipitation of aqueous zirconium salt. However, in the process ofJP-A-2000-327416, when obtaining zirconia fine powder, anti-corrosionmeasures must be taken for the equipment against hydrochloric gas thatis generated when calcinating in order to rapidly dry and calcinate theaqueous solution of zirconium salt and furthermore, a wet pulverizationstep is necessary after calcination. On the other hand, in the processof JP-A-5-246720, because the precipitation behavior when conductinghydrolysis or neutralization precipitation between the zirconiumcompound which is the main component and the yttrium compound which isthe secondary component is usually different, maintaining homogeneity ofthe composition is difficult. Also, the fine particles produced in theaqueous solution agglomerate due to surface tension of the water whendrying and preparing fine particles tend to become difficult. Therefore,when calcinating the obtained dried matter to obtain zirconia rawmaterial powder, the calcinated powder must be pulverized to become fineand uniform.

In this way, the dried matter prepared by spray drying becomes finerparticles if mechanically pulverized further, but in such a case, theprocess becomes a two step process and may be unsatisfactory in terms ofproduction costs. This also applies to dried matter prepared by stilldrying. Also, even when the dried matter is not pulverized, in order toobtain fine particle powder, when drying by spray drying, the liquiddrops containing fine particles of inorganic metal hydrate must be madeas fine as possible using a high pressure nozzle or a high speedrotating atomizer and dried. However, in such a case, clogging of thenozzle, abrasion of the equipment due to high speed rotation of theatomizer and agglomeration of the particles due to surface tension ofwater are difficult to avoid. Furthermore, to suppress the action ofsurface tension of water, additives such as a surfactant must be addedand obtaining inorganic metal hydrate of high purity was difficult.

JP-A-9-175812 discloses the method of preparing a silicic compound bydrying an aqueous solution of a silicic compound by pulse impulse waves.However, this method was considered to shorten the heating anddehydrating time in the heating and calcinating step and reducing theheat load in the calcinating step when heating and calcinating amorphouswater-containing silicic compound powder and does not consider obtainingparticles having sharp particle size distribution.

DISCLOSURE OF INVENTION

The object of the present invention is to provide new inorganic fineparticles and inorganic raw material powder, which suppressagglomeration and adhesion of particles due to heat when preparingpowder from a raw material liquid such as a slurry of zirconia hydratefine particles in order to obtain inorganic raw material powder having asharp particle size distribution, and a process for preparing the same.Furthermore, the present invention aims to provide inorganic fineparticles and inorganic raw material powder, which can make the chemicalstructure homogenous among the produced particles and inside theparticles even in a multi-component system, and a process for preparingthe same.

As a result of intensive studies to solve the above problems, thevarious problems caused by agglomeration of particles in obtaininginorganic raw material powder having a sharp particle size distributionand the various problems in obtaining fine particles having ahomogeneous chemical structure in a multi-component system were found tobe solved by heating and applying impulse waves to a raw materialliquid. Particularly, by using a drying means utilizing impulse wavessuch as a pulse combustion dryer to the raw material liquid, the aboveproblems can be solved without adding an additive such as a surfactant.

That is, the present invention relates to a process for preparinginorganic fine particles, which comprises heating and applying impulsewaves to a raw material liquid.

The impulse waves are preferably ultrasonic vibration.

Heating and application of impulse waves to the raw material liquid arepreferably conducted by contacting the raw material liquid with pulsecombustion gas.

The pulse combustion gas preferably has frequency range of 50 to 1000Hz, pressure amplitude of at least +0.2 kg/cm², sound pressure of 100 to200 decibel and contact gas temperature of 100 to 1000° C.

The raw material liquid is preferably a mixture of a solvent and aninorganic metal compound and/or a solution of an inorganic metalcompound.

The mixture of a solvent and an inorganic metal compound is notparticularly limited as long as the inorganic metal compound isinsoluble in the solvent and is preferably a mixture of a solvent and aninorganic metal hydrate. The obtained inorganic particles are fineparticles of inorganic metal hydrate.

The mixture of a solvent and an inorganic metal hydrate insoluble in thesolvent preferably contains at least one of a slurry of zirconia hydratefine particles, a slurry of ceria hydrate fine particles, a slurry oftitania hydrate fine particles, a slurry of fine particles of a hydratedsilicic compound and a slurry of alumina hydrate fine particles.

The mixture of a solvent and an inorganic metal hydrate insoluble in thesolvent preferably comprises a neutralized hydroxide, a neutralizedcoprecipitated hydroxide, a hydrolyzate or a composite thereof.

The fine particles of inorganic metal hydrate in the mixture of asolvent and an inorganic metal hydrate insoluble in the solvent arepreferably 0.01 to 50 μm.

The solution of an inorganic metal compound preferably is an aqueoussolution of a water-soluble inorganic metal salt and the obtainedinorganic particles are fine particles of inorganic metal salt ormodifications thereof.

The aqueous solution of a water-soluble inorganic metal salt preferablycontains at least one of an aqueous solution of zirconyl chloride, anaqueous solution of zirconyl sulfate, an aqueous solution of zirconylnitrate, an aqueous solution of cerium chloride, an aqueous solution oftitanium tetrachloride, an aqueous solution of titanium trichloride, anaqueous solution of aluminum chloride, an aqueous solution of magnesiumchloride, an aqueous solution of calcium chloride or an aqueous solutionof a silicic compound.

The present invention also relates to a process for preparing inorganicraw material powder, which comprises calcinating and pulverizing theinorganic particles obtained by the above process.

The present invention also relates to inorganic fine particles andinorganic raw material powder having a ratio of arithmetic standarddeviation to arithmetic average size found from particle sizedistribution measured by an optical method of at most 0.8.

According to the present invention, inorganic fine particles can beobtained, which have approximately the same bulk density as inorganicmetal hydrates obtained by the conventional drying method, smallparticle size and extremely sharp particle size distribution, withoutusing an additive such as a surfactant to suppress agglomeration actiondue to surface tension of water. Also, inorganic fine particles can beobtained, which are compositionally homogeneous when microscopicallyobserved, even in a multi-component system. Furthermore, in preparationof inorganic raw material powder the pulverizing step conducted aftercalcination of inorganic fine particles can be conducted in an extremelyshort time.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is the particle size distribution of zirconia hydrate powderobtained by pulse combustion drying, still drying or spray drying ahydrolyzed slurry of zirconia hydrate fine particles.

BEST MODE FOR CARRYING OUT THE INVENTION

The process for preparing inorganic particles of the present inventionis conducted by heating and applying impulse waves to a raw materialliquid. Herein, the raw material liquid is not particularly limited, aslong as the liquid contains the inorganic metal compound of theinorganic particles to be obtained or is a liquid that is converted intothe inorganic particles to be obtained by modification. For example, amixture of a solvent and an inorganic metal compound that is liquid atroom temperature such as various alcoxides including tetraethylorthosilicate and titanium isopropoxide and titanium tetrachloride or asolution in which an inorganic metal compound is dissolved can be used.

A mixture of a solvent and an inorganic metal compound refers to amixture wherein the inorganic metal compound to be obtained is mixed ina solvent as compounds such as hydrates, oxides, carbonates andsulfates. Specific examples of hydrates are zirconia hydrate, ceriahydrate, titania hydrate, hydrated silicic compound and alumina hydrate.Examples of oxides are zirconium oxide, cerium oxide and titanium oxideand examples of carbonates are zirconium carbonate, cerium carbonate andcalcium carbonate. Examples of sulfates are cerium sulfate, aluminumsulfate and magnesium sulfate.

As the solvent, water, ethanol, methyl ethyl ketone and toluene aresuitably used.

Particularly, a slurry of zircona hydrate fine particles, a slurry ofceria hydrate fine particles, a slurry of titania hydrate fineparticles, a slurry of fine particles of hydrated silicic compound and aslurry of alumina hydrate fine particles, which contain inorganic metalhydrate as the inorganic metal compound and water as the solvent, arepreferably used from the viewpoints that the discharged water is easilytreated and special explosion-proof equipment is unnecessary.Particularly, the slurry of zircona hydrate fine particles is preferablefrom the viewpoint of chemical stability.

The inorganic metal compound can be used alone or mixed together withdifferent types.

For example, zirconia hydrate can be hydrate containing zirconium oxideand zirconium hydroxide alone or a mixture thereof and yttrium oxide,yttrium hydroxide, aluminum oxide, aluminum hydroxide, cerium oxideand/or cerium hydroxide (hereinafter referred to as complex zirconiahydrate).

The zirconium oxide or zirconium hydroxide, yttrium oxide or yttriumhydroxide, aluminum oxide or aluminum hydroxide and cerium oxide orcerium hydroxide can be produced by hydrolysis of an aqueous solution ofzirconium salt, yttrium salt, aluminum salt or cerium salt or by addingalkali or ammonia thereto.

Examples of the zirconium salt are zirconium oxychloride, zirconylnitrate and zirconyl sulfate. Also, a mixture of zirconium hydroxide andacid (sulfuric acid, hydrochloric acid and nitric acid) can be used. Ofthese, from the viewpoints of economic efficiency, ease in handling andease in after-treatment, zirconium oxychloride, zirconyl nitrate or amixture of zirconium hydroxide and hydrochloric acid or nitric acid ispreferably used.

Examples of the yttrium salt are yttrium chloride, yttrium nitrate andyttrium sulfate. Also, a mixture of yttrium hydroxide and acid (sulfuricacid, hydrochloric acid and nitric acid) can be used. Of these, from theviewpoints of economic efficiency, ease in handling and ease inafter-treatment, yttrium chloride, yttrium nitrate or a mixture ofyttrium hydroxide and hydrochloric acid or nitric acid is preferablyused.

Examples of the cerium salt are cerium chloride, cerium nitrate andcerium sulfate. Also, a mixture of cerium hydroxide and acid (sulfuricacid, hydrochloric acid and nitric acid) can be used. Of these, from theviewpoints of economic efficiency, ease in handling and ease inafter-treatment, cerium chloride, cerium nitrate or a mixture of ceriumhydroxide and hydrochloric acid or nitric acid is preferably used.

When obtaining high strength zirconia, the content of yttrium oxide ispreferably 1.0 to 10% by weight, more preferably 2 to 6% by weight,further preferably 4.5 to 5.5% by weight based on zirconia. When thecontent is less than 1.0% by weight, stability of tetragonal zirconiaparticles tends to decrease. When the content is more than 10% byweight, the proportion of cubic zirconia particles increase and thetoughness of the sintered article using the particles as the rawmaterial tends to decrease.

The content of cerium oxide is preferably 1.0 to 77% by weight, morepreferably 13 to 68% by weight, further preferably 26 to 68% by weightbased on zirconia. When the content is less than 1.0% by weight,stability of tetragonal zirconia tends to decrease. When the content ismore than 77% by weight, hardness and thermal stability tend todecrease.

Furthermore, the zirconia hydrate can contain aluminum compounds such asaluminum oxide, aluminum hydroxide, aluminum chloride and aluminumsulfate.

The particles size of the fine particles of the complex inorganic metalhydrate in the slurry obtained by mixing the above is preferably 0.01 to50 μm, more preferably 0.05 to 50 μm, further preferably 0.1 to 20 μm,particularly preferably 0.3 to 15 μm. When the particle size is lessthan 0.01 μm, the particle size tends to increase by agglomeration evenwhen pulse combustion drying is conducted. When the particle size ismore than 50 μm, the effect of crushing the particles by pulsecombustion gas becomes small and the particle size after drying tends tobecome large.

The concentration of neutralized hydroxides, neutralized coprecipitationhydroxides, hydrolyzates or composites thereof in the fine particles ofthe complex inorganic metal hydrate in the slurry is preferably 5 to 50%by weight, more preferably 5 to 45% by weight, further preferably 10 to40% by weight, particularly preferably 10 to 30% by weight converted todried matter. When the concentration is less than 5% by weight, theparticle size after drying becomes too fine that collecting tends tobecome difficult. When the concentration is more than 50% by weight,operations such as transporting of the slurry become difficult, theparticle size after drying becomes large and the particle sizedistribution tends to become broad.

Examples of the method for mixing the composite of fine particles ofcomplex inorganic metal hydrate such as zirconia particles are themethod of preparing a mixed solution by dissolving soluble zirconiumsalts and secondary soluble salts (and soluble aluminum salts) in waterand thereafter, hydrolyzing to obtain hydrated zirconia from zirconiumsalts; the method of obtaining hydrated zirconia from zirconium saltsand thereafter neutralizing to obtain complex zirconia hydrate; themethod of obtaining complex zirconia hydrate by neutralizing andcoprecipitating the above mixed solution to produce complex hydroxide;the method of suspending zirconia particles and yttrium oxide particlesand/or cerium oxide particles alone respectively in a solvent and thenmixing; and the method of mixing zirconia particles and yttrium oxideparticles and/or cerium oxide particles and then suspending in asolvent, but are not limited thereto.

Examples of the solvent used when preparing the slurry are water,alcohol, a mixed solution of water/alcohol, a mixed solution of methylethyl ketone/water and toluene. Of these, from the viewpoints ofeconomical efficiency and safety, water or a mixed solution ofwater/alcohol is preferable.

The fine particles of complex zirconia hydrate can be prepared byheating the slurry of fine particles of complex zirconia hydrate andthen contacting with impulse waves.

As the raw material liquid in the process for preparing fine particlesof an inorganic metal compound of the present invention, a solution ofan inorganic metal compound can be used. Examples of the solvent used inthe solution are water, alcohol and a mixed solution of water/alcohol.Of these, from the viewpoints of economical efficiency and safety, wateror a mixed solution of water/alcohol is preferable.

Examples of the inorganic metal compound used in the solution of aninorganic metal compound are hydroxides, chlorides, sulfates, nitratesand alcoxide compounds. Of these, from the viewpoints of economicalefficiency and safety, an inorganic metal salt that is soluble in wateris preferable. Examples of inorganic metal salts soluble in water arechlorides, sulfates and nitrates. Specific examples of the aqueoussolution of an inorganic metal salt are an aqueous solution of zirconylchloride, an aqueous solution of zirconyl sulfate, an aqueous solutionof zirconyl nitrate, an aqueous solution of cerium chloride, an aqueoussolution of titanium tetrachloride, an aqueous solution of titaniumtrichloride, an aqueous solution of aluminum chloride, an aqueoussolution of magnesium chloride and an aqueous solution of calciumchloride. Of these, an aqueous solution of a chloride is preferable fromthe viewpoints of economical efficiency and ease in handling.

In the process for preparing inorganic fine particles of the presentinvention, impulse waves refer to the state in which sudden increase anddecrease of pressure, density and temperature in compressed fluid isrepeated and ultrasonic waves, compression waves that accompanyexplosion and high speed transferring of matter can be used.Particularly, ultrasonic vibration is preferable from the viewpoints ofeconomical efficiency and safety. The means for heating the raw materialliquid is not particularly limited and means such as electric heatingusing a resistance heating element, gas heating by combustion offlammable gas and indirect heating via a jacket can be employed.

As the means for applying impulse waves and heating, contacting withpulse combustion gas is particularly preferable, as both application ofimpulse waves and heating can be achieved simultaneously by a singlemeans.

An example of the pulse combustion system that generates pulsecombustion gas is for example, the drying machine described inJP-A-8-40720. This system is equipped with a pulse combustion chamber, adrying chamber, a cyclone and a bug filter.

Pulse combustion gas refers to combustion gas that usually pulses at acycle of 50 to 1000 times per second. This combustion gas is generatedby the pulse combustion chamber. The raw material liquid transferredinto the combustion gas atmosphere is separated into fine liquid dropshaving sharp particle size distribution and dried instantaneously by thehot air drying effect and the physical impact properties due to thepulse function including sound pressure and air pressure. The mechanismthereof is not clear. However, it is inferred that the impulse waves acton the liquid column of the raw material liquid sprayed from a commonnozzle tip or rotary disc or the surface of the liquid drops after theliquid column is separated and fine liquid drops having sharp particlesize distribution, which cannot be obtained by merely using a sprayingmeans such as a nozzle or a rotary disc, are produced because the liquidcolumn is separated into liquid drops of uniform size or the liquiddrops are reseparated into liquid drops of uniform size due to impact oneach other of the plurality of waves generated on the liquid column orthe surface of the liquid drops. The inorganic fine particles obtainedin this way may be partially modified depending on the type of thematerial, but usually, chemical change of the components do not occurand the homogeneity of the chemical composition as the raw materialliquid is maintained even in the case of a multi-component system.Therefore, the pulse combustion system is effective as a means forapplying impulse waves and heating. The mechanism by which the chemicalhomogeneity is maintained is not clear. It is inferred that one reasonthe homogeneity is maintained is because the transport distance of thesolute component inside the liquid drops is small as the size of theobtained liquid drops are fine for the reason described above andfurthermore, the transport speed of the solute component inside theliquid drops is kept low as the heating temperature is low. However,this explanation is insufficient and the physical impact actionincluding sonic wave force due to rapid pulse action of impulse waves isinferred to be largely involved in the removal of the solvent from theliquid drops.

The frequency range of the pulse combustion gas is preferably 50 to1,000 Hz, more preferably 100 to 900 Hz, further preferably 125 to 550Hz. When the frequency is less than 50 Hz, vibration failure due to lowfrequency may occur. When the frequency is more than 1,000 Hz, theeffect of drying may not sufficiently be obtained.

The pressure amplitude of the pulse combustion gas is preferably atleast ±0.2 kg/cm², more preferably at least ±0.4 kg/cm², furtherpreferably at least ±0.6 kg/cm². When the pressure amplitude is lessthan ±0.2 kg/cm², separation into fine liquid drops is insufficient andthe dispersion effect of the produced particles may be insufficient.

The sound pressure of the pulse combustion gas is preferably 100 to 200decibel, more preferably 120 to 160 decibel, further preferably 140 to150 decibel. When the sound pressure is less than 100 decibel,sufficient stirring action and drying action due to the repeatedpressure reducing action of air caused by sonic waves in the vicinity ofthe dispersed particles may not be obtained. When the sound pressure ismore than 200 decibel, large cost tends to become necessary for soundproofing measures.

The contact gas temperature of the pulse combustion gas is preferably100 to 1000° C., more preferably 150 to 700° C., further preferably 200to 500° C. When the contact gas temperature is lower than 100° C., theparticles may not be sufficiently dried. When the contact gastemperature is more than 1000° C., the particles tend to be moresusceptible to modification by heat.

As the material of the pulse combustion system machine, stainless steelis suitably used from the viewpoints of economical efficiency andmaintenance. In the case that corrosive gas is generated as the liquidraw material dries, the inner face of the drying chamber can be coatedwith resin such as Teflon® and corrosion-resistant ceramics. Whencoating with resin, the flow rate and temperature of the pulsecombustion gas, the flow rate of the liquid raw material and theconcentration of volatile components such as the solvent can be adjustedso that the temperature of the drying chamber is maintained to at mostthe heat resistant temperature of resin such as Teflon®.

The inorganic raw material powder can be prepared by calcinating,pulverizing and when necessary, granulating the inorganic fine particlesobtained by heating and applying impulse waves to the raw materialliquid. For example, zirconia raw material powder can be prepared bycalcinating, pulverizing and granulating fine particles of complexzirconia hydrate obtained by heating and applying impulse waves to aslurry of zirconia hydrate fine particles. Examples of the calcinatingmachine are an electric furnace, a vacuum calcinating furnace, anatmospheric calcinating furnace, a gas furnace and an electromagneticinduction furnace, which can control the temperature increase speed, thetemperature and particularly, when aiming to obtain non-oxide or metalinorganic raw material powder, the calcinating atmosphere, but are notlimited thereto.

The calcination temperature must be a temperature at which conversion tothe target compound is achieved by chemical reaction and solid solutionand furthermore, is selected accordingly so that the particle size andthe degree of agglomeration of the produced particles is within thepreferable range. For example, when obtaining fine particles of complexzirconia oxide from complex zirconia hydrate, the temperature ispreferably 600 to 1100° C., more preferably 800 to 1000° C. When thecalcination temperature is lower than 600° C., solid solution of thezirconium oxide obtained by oxidization of complex zirconia hydrate andthe secondary component is insufficient and furthermore, absorption ofmoisture becomes large and the weight loss by burning when sintering themolded article tends to become large. When the temperature is higherthan 1100° C., the produced particles grow too much that sinteringproperties tend to become poor.

As the atmosphere when calcinating, when oxide raw material powder isdesired, the usual atmospheric pressure atmosphere and combustion gasatmosphere for heating are suitably used and when non-oxide or metal rawmaterial powder is desired, nitrogen gas atmosphere, vacuum atmosphereand inert gas atmosphere such as argon gas are suitably used dependingon the purpose. In such a case, for example when carbide raw materialpowder is desired, calcination can be conducted in the condition of acarbon source being co-present in the calcinating furnace or thesubstance which is to be the carbon source such as carbon or phenolresin can be dissolved or dispersed in advance in the raw materialliquid.

The inorganic fine particles of the present invention have a ratio ofarithmetic standard deviation to arithmetic average size found fromparticle size distribution of secondary particle size measured by anoptical method (hereinafter referred to as “variation coefficient”) ofpreferably at most 0.8, more preferably at most 0.7, further preferably0.5. When the ratio is more than 0.8, particularly in fields whereindemands for precision are rising in recent years, coarse particles andfine particles must be eliminated, when the problem of scattering in theinorganic raw material powder and the ultimate product is caused andwhen the target inorganic fine particles are required to be within aspecific particle size range, and yield may decrease. Also, whenpulverization treatment is conducted in order to prepare raw materialpowder, operations may become complicated, for example, pulverization intwo steps such as pre-treatment or coarse pulverization and finepulverization may be required, as coarse particles and fine particlesare mixed together. The variation coefficient is more preferable thesmaller it is and is preferably substantially 0, but is oftensufficiently about 0.2 to achieve the is object of the presentinvention. The inorganic fine particles of the present invention can bedetermined according to the purpose and from the viewpoint of ease inprocessing into raw material powder, the arithmetic average size of thesecondary particles thereof is preferably 0.1 to 20 μm, more preferably0.2 to 5 μm, further preferably 0.3 to 1 μm.

The inorganic raw material powder of the present invention has variationcoefficient found from particle size distribution of secondary particlesize measured by an optical method of preferably at most 0.6, morepreferably at most 0.5, further preferably at most 0.4. When the ratiois more than 0.6, scattering of the ultimate article comprising thepowder as the raw material may be caused or a separate step for matchingthe particle size becomes necessary and yield decreases, causingoperations to become complicated. The variation coefficient is morepreferable the smaller it is and is preferably substantially 0, but isoften sufficiently about 0.2 to achieve the object of the presentinvention. The arithmetic average size of the secondary particles of theinorganic raw material powder of the present invention is notparticularly limited. The particle size can be controlled bypulverization depending on the object and is preferably 0.01 to 1 μm,more preferably 0.03 to 0.8 μm, further preferably 0.05 to 0.5 μm.

The optical method mentioned above refers to the laser diffractionmethod and an example of the laser diffraction particle sizedistribution measuring machine is SALD-2000 made by ShimadzuCorporation.

The inorganic raw material powder obtained by the preparation process ofthe present invention has sharp particle size distribution and issuitably used for various uses in which uniformity is required.

For example, by calcinating and processing, the zirconia raw materialpowder can be used for cutlery and jigs having high strength andtoughness, a pulverizing ball and various structural members such assliding members, mechanical members and optical communication members.Also, the powder can be added to abrasives and cosmetics withoutcalcinating.

Titanium oxide raw material powder can be used as a photocatalyst.

A solid solution of zirconium oxide and cerium oxide and a mixed powderof the solid solution and aluminum oxide can be used as a carrier of athree way catalyst for automobile exhaust gas.

Hereinafter, the present invention is described in detail by means ofExamples, but the present invention is not limited thereto.

Preparation of Zirconia Hydrate

Hydrolyzed Slurry

An aqueous solution containing 94.2% by mol of -zirconium oxychlorideand 5.8% by mol of yttrium chloride was boiled and hydrolyzed andthereafter, neutralized with 28% ammonia water to obtain a precipitate.The obtained precipitate was washed with deionized water until thewashing fluid no longer became clouded by the sliver nitrate aqueoussolution. The washed precipitate was adjusted to be 20% by weightconverted to dried matter and a hydrolyzed slurry was obtained. The fineparticles of complex zirconia hydrate in the slurry were 0.1 μm.

Neutralized Coprecipitated Slurry

An aqueous solution containing 94.2% by mol of zirconium oxychloride and5.8% by mol of yttrium chloride was neutralized by adding 28% ammoniawater to obtain a precipitate. The obtained precipitate was washed withdeionized water until the washing fluid no longer became clouded by thesliver nitrate aqueous solution. The washed precipitate was adjusted tobe 10% by weight converted to dried matter and a neutralizedcoprecipitated slurry was obtained. The fine particles of complexzirconia hydrate in the slurry were 19.2 μm.

(Calcination and Pulverization of Zirconia Hydrate Fine Particles)

The fine particles of complex zirconia hydrate obtained by drying theslurry of fine particles of complex zirconia hydrate were calcinated at800 to 1000° C. using an electric furnace. The calcinated powder waspulverized for a given time by a continuous ready mill (type SLG-03)made by AIMEX.

(Measurement of Powder Properties)

(1) Measurement Method of Particles

Measurement machine: Laser diffraction particle size distributionmeasurement machine (SALD-2000 made by Shimadzu Corporation) Preparationof measurement sample: The sample was added to a 0.3% aqueous solutionof sodium hexametaphosphate so that the concentration of dried matter is0.2% and then dispersed by placing in a 100 W ultrasonic wave generatingmachine for 2 minutes. Thereafter, the particle size was measured. Fromthe frequency data per each obtained particle size fraction, thevariation coefficient was measured from the following equation.Da = ∑  i(Qi × Xi)/∑  iQi$\sigma = \sqrt{\left\lbrack {\sum\quad{i\left\{ {\left( {{Xi} - {Da}} \right)^{2} \times {{Qi}/100}} \right\}}} \right\rbrack}$R = σ/DaHerein, Da is the arithmetic particle size (μm), Qi is the frequencydistribution value in fraction i (%), Xi is the representative particlesize in fraction i (μm), Σi is the total regarding fraction i, σ is thearithmetic standard deviation (μm) and R is the variation coefficient.(2) BET Specific Surface AreaMeasurement Machine: Gemini 2360 (Made by Shimadzu Corporation)

Measurement gas: nitrogen

Preparation of measurement sample and measurement method: A certainamount of the sample was placed in the sample cell and nitrogen wascirculated. The sample was heated for 30 minutes at 200° C. and thendegassing was conducted. Adsorption gas (nitrogen) was simultaneouslycirculated into the sample cell and the balance cell and the differencein pressure between both cells was detected. The surface area of thesample was calculated.

(3) Measurement of Apparent Specific Gravity of Powder

Measurement Machine: Powder Tester (Made by Hosokawa Micron Corporation)

Preparation of measurement sample and measurement method: the weight ofan empty cup having a certain volume was measured and the sample passedthrough a sieve of 250 μm was dropped in by vibration. The specificgravity was calculated from the weight of the is sample in the cup.

(4) Identification of Crystal Phase

Measurement Machine: X-ray Diffraction Meter RAD-C System (Made byRigaku Corporation)

The integrated intensity was found from the diffraction peak of eachcrystal phase by the powder X-ray diffraction method and each crystalphase was found by applying to the following equation.M = [{Im(111) + Im(11-1)}/{Im(111) + Im(11-1) + Ic + t(111)}] × 100C = [Ic + t(111)/{Im(111) + Im(11-1) + Ic + t(111)}] × [  Ic(400)/{Ic(400) + It(400) + It(004)}] × 100T = 100   − M−  C

Herein, M is the mol % of monoclinic zirconia, C is the mol % of cubiczirconia, T is the mol % of tetragonal zirconia, index m is themonoclinic zirconia, index c is cubic zirconia, index t is tetragonalzirconia and c+t represents both cubic zirconia and tetragonal zirconia.The symbols inside the parentheses are each phase index and I attachedto an index represents the integrated intensity in each phase index ofeach crystal phase.

EXAMPLE 1

The hydrolyzed slurry obtained above was dried by pulse combustion. Asthe pulse combustion drying machine, Hypulcon® 25 model made by PultechCorporation was used. The pulse combustion gas generated by the pulsecombustion drying machine has frequency of 550 Hz, pressure amplitude of±0.8 kg/cm², sound pressure of 145 decibel and contact gas temperatureof 280° C. The slurry was dried under conditions of inlet temperature of190 to 200° C. and outlet temperature of 80° C. and the dried matter wassieved by a 200 mesh stainless steel wire mesh sieve. The particle sizeof the sieved particles was measured. The variation coefficientregarding the particle size distribution of the particles was 0.45.Subsequently, the powder was calcinated at 800° C. and 1000° C. Theparticle size and the BET specific surface area of the calcinatedarticle were measured. Thereafter, the calcinated article was pulverizedfor a given time to obtain a pulverized article. The particle size ofthe pulverized article was measured. The results are shown in Tables 1and 2 and FIG. 1.

EXAMPLE 2

The neutralized coprecipitated slurry obtained above was dried by pulsecombustion. The pulse combustion drying machine and the pulse combustiongas generated thereby were the same as in Example 1. The obtained driedmatter was sieved by a 200 mesh stainless steel wire mesh sieve in theabove manner. The particle size of the sieved fine particles wasmeasured. The variation coefficient was 0.63. Subsequently, the powderwas calcinated at 800° C. and 1000° C. The particle size and the BETspecific surface area of the calcinated article were measured.Thereafter, the calcinated article was pulverized for a given time toobtain a pulverized article. The particle size of the pulverized articlewas measured. The results are shown in Tables 1 and 2.

EXAMPLE 3

The experiment was conducted in the same manner as in Example 1, exceptthat the concentration of the precipitate in the hydrolyzed slurry was40% by weight converted to dried matter. The variation coefficient ofthe particle size of the obtained fine particles was 0.77.

COMPARATIVE EXAMPLE 1

The hydrolyzed slurry obtained above was placed in a stainless steelpallet. The water content was evaporated with a hot air dryer at 105° C.and the slurry was still dried. After drying, the dried matter waspulverized with a mortar and sieved with a 200 mesh stainless steel wiremesh sieve. The particle size of the sieved powder was measured. Thevariation coefficient of the obtained powder was 0.91. Subsequently, thepowder was calcinated at 800° C. and 1000° C. The particle size and theBET specific surface area of the calcinated article were measured.Thereafter, the calcinated article was pulverized for a given time toobtain a pulverized article. The particle size of the pulverized articlewas measured. The results are shown in Tables 1 and 2 and FIG. 1.

COMPARATIVE EXAMPLE 2

The hydrolyzed slurry obtained above was spray dried. As the spraydrying machine, L-12 type made by Ogawara MFG. Co., LTD. was used. Theslurry was dried under conditions of inlet temperature of 180° C. andoutlet temperature of 80° C. using a binary nozzle and the dried matterwas sieved by a 290 mesh stainless steel wire mesh sieve. The particlesize of the sieved powder was measured. The variation coefficient was1.01. Subsequently, the powder was calcinated at 800° C. and 1000° C.The particle size and the BET specific surface area of the calcinatedarticle were measured. Thereafter, the calcinated article was pulverizedfor a given time to obtain a pulverized article. The particle size ofthe pulverized article was measured. The results are shown in Tables 1and 2 and FIG. 1.

COMPARATIVE EXAMPLE 3

The neutralized coprecipitated slurry obtained above was still dried.The drying method was the same as in Comparative Example 1. Thevariation coefficient of the particle size of the obtained fineparticles was 0.95.

COMPARATIVE EXAMPLE 4

The neutralized coprecipitated slurry obtained above was spray dried.The drying machine and method were the same as in Comparative Example 2.The variation coefficient of the particle size of the obtained fineparticles was 0.98.

As additional examples, Examples using an aqueous solution of zirconylchloride are shown below.

EXAMPLE 4

(Preparation of Zirconyl Chloride Aqueous Solution)

6252.8 g of zirconium oxychloride octahydrate having purity of 95%, 702g of an aqueous solution of yttrium chloride hexahydrate containing18.35% of yttrium converted to yttrium oxide and 34.2 g of ammoniumchloride hexahydrate of a high grade reagent were dissolved in 12.135 kgof deionized water while stirring.

The aqueous solution of zirconium salt obtained in the above manner wastreated with a pulse combustion gas impulse wave system. As the pulsecombustion gas impulse wave system, Hypulcon HP-2 made by PultechCorporation was used. The pulse combustion gas generated by the pulsecombustion gas impulse wave system has frequency of 900 Hz, pressureamplitude of ±0.25 kg/cm², sound pressure of 120 decibel and contact gastemperature of 150° C. The aqueous solution was dried under conditionsof treatment chamber and outlet temperature of 70° C. and the obtainedpowdery solid particles were thermally treated for 1 hour at 550° C. and1000° C. to obtain high strength zirconium oxide powder. When the metalatoms are not uniformly dispersed as in the prior art, when payingattention to dispersion of yttrium, the diffraction peak of monocliniczirconia is obtained from the area of the powder wherein a small amountof yttrium is present or yttrium is absent. However, the crystal phaseof the powder obtained in the present Example was a tetragonal singlephase regardless of the temperature at which thermal treatment wasconducted and the diffraction peak of monoclinic zirconia is notexpressed. Also, the obtained powder was found to have a uniformchemical composition when microscopically observed.

COMPARATIVE EXAMPLE 5

The same aqueous solution of zirconium salt as in Comparative Example 4was spray dried at a drying temperature of 110° C. using Spray DrierSD-2 made by Tokyo Rikakikai Co., LTD. The obtained dried particles werethermally treated for 1 hour at 550° C. to obtain high strengthzirconium oxide powder. The crystal phase of the obtained powder wastetragonal and monoclinic and the amount of monoclinic zirconia was21.8% by mol. Consequently, the obtained powder had areas of differentchemical composition microscopically and was inferior in homogeneity.

EXAMPLE 5

(Preparation of Zirconyl Chloride Aqueous Solution)

Zirconium oxychloride octahydrate (containing 5% by weight of freehydrogen chloride) was dissolved in distilled water to prepared anaqueous solution of 1.5% by mol/liter. The aqueous solution was driedusing the small-size pulse combustion gas impulse wave system, HypulconHP-2 made by Pultech Corporation. The pulse combustion gas generated bythe pulse combustion gas impulse wave system has frequency of 125 Hz,pressure amplitude of ±0.5 kg/cm², sound pressure of 160 decibel andcontact gas temperature of 500° C. and treatment was conducted at anaverage treatment rate of 1.5 liter/hour at drying chamber temperatureof 60° C. The water content and the chlorine content of the obtainedpowder were analyzed and the water concentration was 16.9% and thechlorine concentration was 32.8%. That is, although the dryingtemperature is lower than 100° C., it is observed that crystal water andpart of the flocked chlorine are removed from the raw materialZrOCl₂.8H₂O and powder estimated to have the chemical structure ofZrO_(1.01)Cl_(1.98).2H₂O is obtained.

COMPARATIVE EXAMPLE 6

The same aqueous solution as in Example 3 was spray dried at a dryingtemperature of 110° C. using Spray Drier SD-2 made by Tokyo RikakikaiCo., LTD. The water content and the chlorine content of the obtainedpowder were 44.7% and 22.0% respectively. That is, although drying wasconducted at a temperature higher than 100° C., substances other thanfree water and free hydrogen chloride are not removed and the obtainedpowder has the same chemical structure as the raw material, ZrOCl₂.8H₂O.TABLE 1 Particle size of zirconia hydrate and BET specific surface areaafter calcination (before pulverization) Particle size of Calcinationtemperature zirconia hydrate 800° C. 1000° C. particles before BET BETcalcination (μm) specific specific Particle Variation surface surfaceApparent size coefficient Particle area Particle area specific gravity(μm) (−) size (μm) (m²/g) size (μm) (m²/g) (g/ml) Ex. 1 5.3 0.45 4.525.6 4.2 14.9 0.796 Ex. 2 15.4 0.63 18.6 13.6 21.0 5.6 0.753 Ex. 3 11.70.77 13.0 21.3 12.8 14.0 0.793 Com. 19.1 0.91 20.3 31.7 25.0 15.6 0.592Ex. 1 Com. 11.8 1.01 10.8 30.6 9.3 15.6 0.820 Ex. 2 Com. 38.2 0.95 38.321.8 31.9 7.5 0.777 Ex. 3 Com. 21.7 0.98 22.1 19.0 25.0 5.6 0.773 Ex. 4

TABLE 2 Calcination temperature Pulverization 800° C. 1000° C. time 0min. 5 min. 30 min. 60 min. 0 min. 5 min. 30 min. 60 min. Ex. 1 4.5 0.60.2 0.2 4.2 0.4 0.2 0.2 Ex. 2 18.6 0.7 0.5 0.4 21.0 0.4 0.3 0.3 Ex. 313.0 0.9 0.5 0.5 12.8 0.6 0.4 0.4 Com. Ex. 1 20.3 3.5 0.9 0.6 25.0 2.50.4 0.3 Com. Ex. 2 10.8 0.7 0.5 0.5 9.3 1.5 0.3 0.2 Com. Ex. 3 38.3 15.01.2 0.4 31.9 13.2 0.6 0.4 Com. Ex. 4 22.1 8.6 0.5 0.4 25.0 9.5 0.5 0.3

FIG. 1 shows that by drying the fine particles of complex zirconiahydrate in the slurry by the pulse combustion gas of the presentinvention, the agglomeration action due to surface tension of water canbe suppressed without adding an additive such as a surfactant and theparticle size of the dried matter can be made small. Also, the particleshave extremely sharp particle size distribution.

Table 1 shows that the bulk density of the zirconia hydrate fine powderobtained by the present invention is about the same as particles ofzirconia hydrate powder obtained by still drying or spray drying. Also,the particle size of the zirconia hydrate fine particles obtained by thepresent invention is about at most 5 μm when the hydrolyzed slurry isused and about at most 20 μm when the neutralized coprecipitated slurryis used.

Also, in pulse combustion drying, clogging of the nozzle and abrasion ofthe equipment due to high speed rotation did not occur.

Table 2 shows that in the pulverization step conducted after calcinationof the fine particles of zirconia hydrate obtained by the presentinvention, the pulverization time for making the particle size uniformcan be extremely short.

From comparing Example 4 and Comparative Example 5, it is evident thatin the high strength zirconium oxide powder obtained by the presentinvention, the ratio of yttrium oxide to zirconium oxide within aspecific range both macroscopically and microscopically and therefore,the powder is powder having extremely high chemical homogeneity.

From comparing Example 5 and Comparative Example 6, it is evident thatcrystal water and part of the flocked chlorine which usually cannot beremoved unless exposed to a high temperature are removed from the fineparticles of zirconium oxychloride obtained by the present invention,although treatment is conducted at a temperature lower than in theComparative Example. This implies that free water which has weakerbonding force than crystal water is already removed at a temperaturelower than in the Example. The reason therefor is presumed to be thestirring effect of the surface of the liquid drops due to sudden pulseaction of impulse waves or the evaporation accelerating effect of freewater due to low pressure areas in pressure fluctuation, but is notclear. According to the preparation process of the present invention,synthesis of powder at a temperature lower than the usual method ispossible. By using the method of the present invention, even when highlycorrosive substances such as strongly acidic substances given inExamples 4 and 5 are used, the heating temperature can be low andtherefore, resin such as Teflon® can be used. As a result, the range ofchoice of the machine material is wide, thus being economical.

INDUSTRIAL APPLICABILITY

The present invention can be applied to synthesis of electronic ceramicsmaterial such as barium titanate, synthesis of 20 titanium oxidephotocatalysts, three way catalyst carriers for automobile exhaust gasand non-oxide ceramics material such as nitrides, carbides and borides,metal materials used in powder metallurgy and fillers used in resin andfiber. The range of application thereof is not limited thereto.

1. (canceled)
 2. (canceled)
 3. (canceled)
 4. A process for preparinginorganic fine particles, which comprises heating and applying impulsewaves to a raw material liquid, wherein heating and application ofimpulse waves to said raw material liquid is conducted by contactingsaid raw material liquid with pulse combustion gas, and said pulsecombustion gas has frequency range of 50 to 1000 Hz, pressure amplitudeof at least ±0.2 kg/cm², sound pressure of 100 to 200 decibel andcontact gas temperature of 100 to 1000° C.
 5. The process for preparinginorganic fine particles of claim 4, wherein said raw material liquid isa mixture of a solvent and an inorganic metal compound and/or a solutionof an inorganic metal compound.
 6. The process for preparing inorganicfine particles of claim 5, wherein said mixture of a solvent and aninorganic metal compound is a mixture of a solvent and an inorganicmetal hydrate insoluble in said solvent and the obtained inorganicparticles are fine particles of inorganic metal hydrate.
 7. The processfor preparing inorganic fine particles of claim 6, wherein said mixtureof a solvent and an inorganic metal hydrate insoluble in said solventcontains at least one of a slurry of zirconia hydrate fine particles, aslurry of ceria hydrate fine particles, a slurry of titania hydrate fineparticles, a slurry of fine particles of a hydrated silicic compound anda slurry of alumina hydrate fine particles.
 8. The process for preparinginorganic fine particles of claim 6, wherein said mixture of a solventand an inorganic metal hydrate insoluble in said solvent comprises aneutralized hydroxide, a neutralized coprecipitated hydroxide, ahydrolyzate or a composite thereof.
 9. The process for preparinginorganic fine particles of claim 6, wherein said fine particles ofinorganic metal hydrate in said mixture of a solvent and an inorganicmetal hydrate insoluble in said solvent are 0.01 to 50 μm.
 10. Theprocess for preparing inorganic fine particles of claim 5, wherein saidsolution of an inorganic metal compound is an aqueous solution of awater-soluble inorganic metal salt and the obtained inorganic particlesare fine particles of inorganic metal salt or modifications thereof. 11.The process for preparing inorganic fine particles of claim 10, whereinsaid aqueous solution of a water-soluble inorganic metal salt containsat least one of an aqueous solution of zirconyl chloride, an aqueoussolution of zirconyl sulfate, an aqueous solution of zirconyl nitrate,an aqueous solution of cerium chloride, an aqueous solution of titaniumtetrachloride, an aqueous solution of titanium trichloride, an aqueoussolution of aluminum chloride, an aqueous solution of magnesiumchloride, an aqueous solution of calcium chloride or an aqueous solutionof a silicic compound.
 12. A process for preparing inorganic rawmaterial powder, which comprises calcinating and pulverizing theinorganic particles obtained by the process of claim
 4. 13. An inorganicfine particle having a ratio of arithmetic standard deviation toarithmetic average size found from particle size distribution ofsecondary particle size measured by an optical method of at most 0.8.14. An inorganic raw material powder having a ratio of arithmeticstandard deviation to arithmetic average size found from particle sizedistribution of secondary particle size measured by an optical method ofat most 0.6.
 15. The inorganic fine particle of claim 13, wherein saidarithmetic average size of secondary particles is 0.1 to 20 μm.
 16. Theinorganic raw material powder of claim 14, wherein said arithmeticaverage size of secondary particles is 0.1 to 1 μm.